1. Field of the Invention
This invention relates to the catalytic methanation of carbon oxides such as carbon monoxide.
2. Description of the Prior Art
In the fixed bed catalytic methanation of gases containing carbon monoxide and hydrogen, the reaction between the carbon monoxide and hydrogen is very exothermic and if not controlled within the reactor can cause overheating of the catalyst and/or thermal cracking of the product methane to carbon and hydrogen. Carbon formation through thermal cracking in turn has a tendency to foul the catalyst and plug up the gas passages in the catalyst bed. Also, it is important that the gas enter at a temperature sufficient to initiate the catalytic reaction of the carbon monoxide with hydrogen to form methane and to prevent the formation of a carbonyl compound which can occur through the reaction of the carbon monoxide with the catalyst at temperatures below proper operating temperatures.
To overcome some of these problems caused by overheating or carbonyl formation, a hot gas recycle method was developed by the U.S. Bureau of Mines (Industrial and Engineering Chemistry, Product Research and Development, 3, 150 (1964)). Also, direct cold gas recycle and internal cooling of the reactor by heat transfer surfaces within the bed are recognized methods by which temperature controls may be effected. Local heating is difficult to avoid when using the latter and the building of internal exchange surfaces tends to be expensive. The hot gas recycle and direct cold gas recycle methods on the other hand require high recycle ratios. As a consequence large pressure drops through the catalyst beds occur and the attendant requirement for compressor power is increased proportionately. In addition, despite the use of high recycle ratios there nevertheless is produced a "tail gas" of carbon oxides which indicates that conversion is not complete.